Conventional anodized aluminum coatings contain pores with diameters of 10-20 nm that are present at very high density, ca. 10.sup.10 cm.sup.-2. The pores are generally aligned normal to the metal surface. These pores extend through the coating thickness, with a thin "barrier" oxide, typically 10-20 nm thick, at the pore base, and, depositing material into the pores of anodic alumina in order to change the coating properties is known in the art. For example, filling with a fluorinated hydrocarbon provides lubricity, and imbibing dye into the pores can make an attractive colored surface. Depositing a small amount of certain metals into each pore creates attractive shades from gold to bronze by a light scattering phenomena. This is widely practiced commercially and is known as electrolytic coloring. This electrolytic coloring process consists generally of two steps: first, dc anodization to grow the porous oxide, for example, in sulfuric acid; and, second, an ac electrolysis in a bath containing the metal cation to be deposited. A general review of electrolytic coloring is given in chapter 8 of Vol. 1 of Wernick, Pinner and Sheasby, "The Surface Treatment and Finishing of Aluminum and its Alloys, 5th ed.". Moreover, U.S. Pat. No. 3,382,160 issued to T. Asuda on May 7, 1968, and U.S. Pat. No. 4,431,489 issued to B. R. Baker, R. L. Smith and P. W. Bolmer on Feb. 14, 1984 are examples of prior art teachings of electrolytic coloring.
Whether or not a substance is deposited in the coating pores, it is common practice to "seal" the coating by reaction with hot water, or to "cold seal" in certain chemical baths. This step is described in Chapter 11, Vol. 2 of the above referenced work by Wernick, Pinner and Sheasby. These reactions cause the coating to swell into the pores and to make it impervious to penetration by ambient atmosphere and more resistant to corrosion.
In the prior art, the pores have been used as templates to make "nano-wire arrays" by electrolytic deposition of metal or semiconductor into the pores. In this application, the deposit in a pore serves as a "wire" of a length equal to the coating thickness. The coating may either be retained as a support for the deposit or dissolved to expose the nano-wires. This is described in a paper by Routkevitch et al, IEEE Trans. Electr. Dev. 43, 1646-58 (1996).
It has been found difficult to electrolytically deposit another oxide into the pores because this requires anodic conditions which will generally result in further growth of anodic aluminum oxide. For example, Baba, Yoshino and Kono (Adv. Metal Finishing Technology in Japan-1980, p. 129) found that deposition of a small amount of gold into the pores blocked anodic oxidation of aluminum during a subsequent anodic deposition of electrochromic tungsten oxide. In this way they created a layer that changed color in response to a change in voltage polarity. In order to get the strongest color change it would be necessary to fill all, or a majority, of the pores with the electrochromic oxide.
Japanese Patent JP 60,165,391 (Aug. 28, 1985) teaches electrolytically coloring anodized aluminum by directly depositing metal oxides into the pores. This reference also teaches using cathodic dc with solutions containing salts of the metal cation to be deposited, and ac with solutions containing oxyanions of the metal (oxide) to be deposited.
Anodized aluminum is widely used as the exterior surface for spacecraft because it is lightweight, easily fabricated, provides abrasion and corrosion resistance, and can be made to have a range of useful optical properties, described in terms of the coating absorptance and emittance. In a space environment the coating has a typical resistivity of 10.sup.14 ohm cm (negative bias voltage on substrate). This creates a problem during operation because an electrical charge from the space plasma builds up on the surface and cannot bleed off through this highly insulating coating. High voltages (&gt;100 V) may develop across the coating which result in arcing and sporadic discharge with a frequency that depends on details of orbit, bias voltage and location on the spacecraft. The discharges and electrical noise interfere with communication and may cause structural damage.